The development of ultra-high modulus polymers has been pursued in many academic and industrial laboratories by the preparation of anisotropic polymer morphologies of highly oriented and extended molecular chains.
It has been known for a long time that the theoretical tensile modulus of a polymer should approach the modulus of steel (.about.208 GPa). However, until a decade ago, the theoretical calculations (for polyethylene .about.240 GPa) were considered unlikely to be achieved, because all known polymers had moduli two orders of magnitude lower. The reason for such a low modulus was that the polymer assumed a random entangled and twisted configuration which had a low load bearing capacity. In recent years, it was realized that the greatest modulus and strength would result from an anisotropic structure of highly oriented, extended, and densely packed chains. Indeed, some polymers, for example polybenzamide and polyethylene, have been processed into fibers that exhibit moduli of 100-200 GPa, thereby indicating that the earlier theoretical values can be approached.
The development of ultra-high modulus products is of paramount importance in view of their significantly lower density; for example, steel is about eight times more dense than polyethylene. The term "specific modulus" refers to the quotient of modulus divided by density; therefor the specific modulus of polyethylene ultra-high modulus fibers is significantly higher than the specific modulus of steel.
Conventional flexible chain polymers, e.g., polyethylene have been processed into high modulus products by processes that may cause a permanent deformation of the internal structure, namely, the conversion of an initially isotropic and spherulitic structure to a fibrillar structure. The fibrils are made of oriented and extended molecular chains which ensure mechanical connection between crystals and load transfer.
Thus, it can be realized that, for maximum mechanical performance, all polymer chains should be extended along the deformation direction. Thus macroscopic deformation, which involves molecular deformation and is accompanied by drastic dimensional changes in the case of flexible polymers, should not be confused with the shaping processes which in general are also accompanied by dimensional changes but do not involve the transformations of a spherulitic to a fibrillar morphology, which, in the case of high density polyethylene, takes place at a deformation ratio of approximately 4. Nor should macroscopic deformation be confused with the conventional melt extrusion process which may involve some molecular orientation. Indeed, during any melt processing operation some molecular orientation is bound to occur because of the viscoelastic nature of polymeric materials. However, the fraction of extended chains is exceedingly small, too small to result in high modulus/strength performance.
Furthermore, the macroscopic deformation described in this specification is not confined, as to deformation limits, to the natural draw of a particular polymer, for such limits can be overcome by the process of the present invention.
Shaping processes such as calendering or rolling are small deformation processes which do not result in morphological transformation necessary for the ultra-high modulus and strength performance and almost unequivocally involve biaxial flow, i.e., deformation in both the longitudinal (machine) and the transverse direction. Rolling combined with stretching may result in uniaxially deformed polymer structures with significantly enhanced tensile properties. However, this technology is confined to the processing of thin sheets and is limited by the excessive loads involved to offset the counter-force and the friction between the roll and the polymer surface.
Anisotropic polymer morphologies with ultrahigh modulus and strength have been obtained by processing conventional flexible chain polymers by solid state deformation using the extrusion and drawing techniques, by extrusion of supercooled melts and by drawing from gels and dilute flowing solutions.
Various semicrystalline polymers have been studied. High-density polyethylene has been studied the most because of its simple composition and its high theoretical modulus (approximately 240 GPa). Similar anisotropic morphologies have been obtained by the chemical construction of polymers with rigid and semirigid backbone chains by introducing para-substituted aromatic units and then processing with solution and melt processes. The para-benzamide polymers and the copolyesters of poly(ethylene terephthalate) and p-acetoxybenzoic acid are examples of rigid and semi-rigid polymers sought to be processed into ultra-high modulus products, i.e., products in which the molecules are not only oriented but are also extended.
Typically, the ultra-high modulus products from the above processes have been in the form of fibers and thin films, that is, structures which do not have bulk mechanical properties. Two recent developments of ultra-high modulus products with bulk structure have been obtained by injection molding of high density polyethylene and the copolyester of poly(ethyleneterephthalate) and p-acetoxybenzoic acid.
The solid-state extrusion process has also been investigated for its potential use for the production of ultra-high modulus products with bulk structure, but it has been severely restricted by low processing rates (a few centimeters per minute), for it is a solid-state deformation process through a convergent geometry. It has also required very high extrusion pressures, especially for the preparation of products with complex or large cross-sectional areas. An analysis of the extrusion process shows that a high extrusion pressure is required (a) to shear and elongate the polymer and b) to overcome the die-polymer friction. Equation (1) shows the pressure balance in the solid-state extrusion process through a conical die: ##EQU1## where P.sub.E is the pressure of extrusion,
P.sub.0 is the pressure at the die exit, PA1 B is .mu.cot [a], PA1 .mu. is the friction coefficient, PA1 a is the die half angle, PA1 .epsilon. is the strain, PA1 .epsilon..sub.0 is the strain at the exit of the die, PA1 .sigma.(.epsilon.) is the true stress at strain .epsilon., PA1 S is the work of shear and shear yield at strain .epsilon..sub.0 and die angle a.
Equation (1) indicates that the friction coefficient term B is significant and that extrusion pressure increases with increasing friction.